Dyes which have been capable of sensitizing silver halide emulsions to infrared regions of the electromagnetic spectrum have been known for many years. Merocyanine dyes and cyanine dyes, particularly those with longer bridging groups between cyclic moieties, have been used for many years to sensitize silver halide to the infrared. U.S. Pat. Nos. 3,619,154, 3,682,630, 2,895,955, 3,482,978, 3,758,461 and 2,734,900; and GB Pat. Nos. 1,192,234 and 1,188,784 disclose well-known classes of dyes which sensitize silver halide to portions of the infrared region of the electromagnetic spectrum. U.S. Pat. No. 4,362,800 discloses dyes to sensitize inorganic photoconductors to the infrared, and these dyes are also effective sensitizers for silver halide.
With the advent of lasers, and particularly solid state laser diodes emitting in the infrared region of the electromagnetic spectrum (e.g., 780 to 1500 nm), the interest in infrared sensitization has greatly increased. Many different processes and articles useful with laser diodes have been proposed. U.S. Pat. No. 4,011,083 discloses photographic elements having a defined pAg, infrared spectral sensitizing methine dyes of defined polarographic cathodic halfwave potential and silver complexing azaindenes, said elements having higher speed in the spectrally sensitized region. U.S. Pat. No. 4,416,522 proposes daylight photoplotting apparatus for the infrared exposure of film. This patent also generally proposes a film comprising three emulsion layers sensitized to different portions of non-visible portions of the electromagnetic spectrum, including the infrared. The film description is quite general and the concentration of imagewise exposure on each layer appears to be dependent upon filtering of the radiation by the apparatus prior to its striking the film surface. U.S. Pat. No. 4,619,892 describes a photographic element capable of providing full color images without exposure to corresponding visible radiation, said element comprising at least three silver halide emulsion layers on a substrate, each associated with different photographic color image forming materials and sensitized to three different portions of the electromagnetic spectrum with at least two layers sensitized to different regions of the infrared region of the electromagnetic spectrum.
It is also known that the addition of specific organic compounds to a silver halide photographic material in addition to the spectrally sensitizing dyes can increase the spectrally sensitized speed of the emulsion by more than one order of magnitude. This is known as a supersensitizing effect. As organic compounds for supersensitization of infrared sensitized silver halide emulsions which are conventionally known, there are illustrated, for example, triazine derivatives described in U.S. Pat. Nos. 2,875,058 and 3,695,888, mercapto compounds described in U.S. Pat. No. 3,457,078, thiourea compounds described in U.S. Pat. No. 3,458,318, pyrimidine derivatives described in U.S. Pat. No. 3,615,632, azaindene compounds described in U.S. Pat. No. 4,011,083, triaryl compounds described in U.S. Pat. No. 4,578,347, thiazolium and oxazolium salts described in U.S. Pat. No. 4,596,767, combinations of supersensitizers described in U.S. Pat. No. 4,603,104 and thiatriazoles described in U.S. Pat. No. 4,780,404.
Photographic elements comprising silver halide emulsion layers sensitized to infrared regions of the electromagnetic spectrum, in particular color photographic elements associated with photographic color image forming materials, are generally liable to undergo changes in sensitivity if stored under different conditions of humidity and temperature. Such change in photographic sensitivity is a critical problem in the use of infrared sensitized photographic materials. It is well known that commercially available photographic materials having sensitivity in the infrared region are unstable in sensitivity and require special caution for preservation thereof, such as storage in a refrigerator. Conventionally known stabilizers such as 1-phenyl-5-mercaptotetrazole are not effective for improving stability of infrared sensitized photographic materials. Therefore, a need exists to develop a technique wich specifically improves the stability on storage of infrared sensitized emulsions.
Mercaptotetrazoles are generally disclosed in U.S. Pat. Nos. 3,266,897 (carboxysubstituted mercapto tetrazoles for use as antifoggants in silver halide emulsions) and 3,397,987 (heterocyclic nitrogen compounds containing a mercapto function for use as development fog inhibitors in silver halide emulsions comprising unfogged surface latent image grains and fogged internal image silver halide grains).
U.S. Pat. No. 3,637,393 describes the use of mercaptotetrazoles in combination with certain hydroquinone compounds to reduce fog and increase speed in silver halide color photographic emulsions.
U.S. Pat. No. 3,457,078 describes the use of mercapto substituted oxazine, oxazole, thiazole, thidiazole, imidazole, or tetrazole, the mercapto substituted compound further containing an electronegative substituent, as supersensitizers and antifoggants in combination with certain cyanine dyes. 1-Phenyl-2-mercaptotetrazole is a suitable substance disclosed in said patent.
Japanese Pat. Appln. No. J0 1013-539 describes the combined use of mercaptotriazole derivatives and mercaptotetrazole derivatives as supersensitizers in infrared sensitive silver halide materials.
U.S. Pat. No. 4,603,104 describes the combined use of arylmercaptotetrazoles and others supersensitizers to increase the speed of spectrally sensitized silver halide photographic emulsions.